I'm pretty sure that my white molly is preganent but I can't be sure. She's been this way for a looong time, over a month. How long does it usually take for her to give birth? Is eating at a derriere part of mating, too?

Well there are the "balloon" mollies and other balloon fish that are extremely overweight looking. I think they're regular fish that were overfed so much before you got it that continued to stay fat. I have 2 balloon tetras and one actually lost weight and looks normal. The other one goes on feeding frenzies everyday.